Method and apparatus for transmitting and receiving a preamble for synchronization in a MIMO-OFDM communication system

ABSTRACT

A method and apparatus for transmitting a preamble for frame synchronization and channel estimation in a MIMO-OFDM communication system are provided. An OFDM communication system using Q transmit antennas generates a base preamble sequence including a CP and an orthogonal sequence. If Q≦a predetermined number M, a preamble sequence for a kth antenna is S(t−(k−1)T/M). If Q&gt;M and k≦M, the preamble sequence transmitted for the kth antenna is S(t−(k−1)T/M). If Q&gt;M and k&gt;M, the preamble sequence for the kth antenna is (−1) (PS-1) S(t−(k−M−1)T/M). Here, S(t) is the orthogonal sequence, T is the period of the orthogonal sequence, and PS is an index indicating a transmission period of the preamble sequence. The preamble sequences are at least twice transmitted from the Q transmit antennas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/965,087, filed Oct. 14, 2004, now U.S. Pat. No. 7,702,028 whichclaims priority under 35 U.S.C. §119 to an application entitled “Methodof Transmitting Preamble for Synchronization in a MIMO-OFDMCommunication System” filed in the Korean Intellectual Property Officeon Oct. 16, 2003 and assigned Serial No. 2003-72176, the contents ofeach of which is incorporated herein by reference in its entirety forall purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a multi-inputmulti-output-orthogonal frequency division multiplexing (MIMO-OFDM)communication system, and in particular, to a method and apparatus fortransmitting a preamble for frame synchronization.

2. Description of the Related Art

OFDM is widely considered an essential transmission scheme fornext-generation wireless communications for its simple implementation,robustness against multi-channel fading, and its capability ofincreasing the data rate through parallel transmission of data signalsat frequencies called sub-carriers. The sub-carriers are mutuallyorthogonal to avoid inter-carrier interference. Their spectrums areoverlapped so that the sub-carriers are spaced from each other with aminimum gap.

An OFDM system is sensitive to errors or offsets including a frequencyoffset, timing errors in a frame or a symbol, and non-linearity causedby a high peak-to-average power ratio (PAPR). Some OFDM systems utilizea coherent detection rather than differential modulation anddemodulation in order to achieve an additional signal-to-noise ratio(SNR) gain of about 3 dB. Their performance depends considerably onwhether or not channel state information (CSI) is available.

The use of multiple transmit/receive antennas further improvescommunication quality and throughput in an OFDM system. This OFDM systemis called a MIMO-OFDM system which is distinguished from a single-inputsingle-output (SISO)-OFDM system.

The MIMO-OFDM system can simultaneously transmit data on a plurality ofsub-channels in the space domain irrespective of whether or not atransmitter requires the CSI. The sub-channels refer to radio paths froma plurality of transmit antennas to a plurality of receive antennas.Thus, the MIMO-OFDM system offers a higher data rate than the SISO-OFDMsystem.

Typically, the MIMO/SISO-OFDM system requires frame synchronization inboth time and frequency and estimation of channel parameters and noisechanges. For the synchronization and estimation, a preamble sequence(i.e. training symbols or a training sequence) is used.

FIG. 1 illustrates the structure of an OFDM frame including a preamblesequence in a typical OFDM communication system. Referring to FIG. 1,the preamble sequence consists of special symbols added as a prefix tothe OFDM frame. In general, the structure and contents of the preambleare known between a transmitter and a receiver. The preamble is soconfigured as to have a relatively low complexity and offer a maximumperformance in the synchronization and estimation process.

An ideal preamble configuration satisfies the following requirements:

(1) Excellent compensation for timing synchronization;

(2) Low PAPR for high-power transmission;

(3) Feasibility for channel estimation;

(4) Feasibility for frequency offset estimation over a wide range; and

(5) Low computation complexity, low overhead and high accuracy.

A description will be made below of conventional preamble structures forMIMO-OFDM frame synchronization and channel estimation.

A first known preamble transmitting/receiving scheme for MIMO-OFDM framesynchronization transmits the same information sequence through alltransmit antennas.

The MIMO-OFDM system must have excellent properties in time-domainperiodic auto-correlation of sequences as well as in cross-correlationof sequences transmitted from different transmit antennas. Idealauto-correlation and cross-correlation properties are determined byEquation (1) and Equation (2), respectively:

$\begin{matrix}{{\phi(k)} = {{\sum\limits_{n = 0}^{N - 1}{s_{q,n}^{*} \cdot s_{q,{({n + k})}_{N}}}} = \{ \begin{matrix}1 & {k = 0} \\0 & {k \neq 0}\end{matrix} }} & (1) \\{{{\Psi(k)} = {{\sum\limits_{n = 0}^{N - 1}{s_{q,n}^{*} \cdot s_{q^{\prime},{({n + k})}_{N}}}} = {0\mspace{14mu}{for}\mspace{14mu}{all}\mspace{14mu} k}}},{q \neq q^{\prime}}} & (2)\end{matrix}$where superscript * denotes a conjugate operator, N denotes the lengthof sequences, q and q′ denote indexes of transmit antennas, and s_(q,n)denotes an nth data symbol in a sequence of length N transmitted from aqth transmit antenna. A sequence that satisfies Equation (1) is anorthogonal sequence. Here, subscript N denotes the period of thesequence.

In an ideal situation a space-time matrix for sequences transmitted fromN transmit antennas is a unit matrix. However, this is impossible in itsapplication because the number of the transmit antennas must be equal tothe length of the sequences.

In the first preamble transmitting/receiving scheme, a preamble sequenceis designed for frame synchronization by copying a predeterminedorthogonal sequence designated for a first antenna to be used for theother antennas, and is represented bys_(q,n)=s_(n) for all q  (3)

A distinctive shortcoming of the above scheme is that SNR may be verylow in the case of a correlated channel. For a 2×2 MIMO system using twotransmit antennas and two receive antennas, for instance, a receivedsignal is expressed as

$\begin{matrix}{{r_{j}\lbrack {n,k} \rbrack} = {{\sum\limits_{i}{{H_{ij}\lbrack {n,k} \rbrack}{S\lbrack {n,k} \rbrack}}} + {n_{j}\lbrack {n,k} \rbrack}}} & (4)\end{matrix}$where r_(j)[n, k] denotes a frequency-domain signal received at a jthreceive antenna, n_(j)[n, k] denotes white Gaussian noise, H_(ij)denotes a channel response from an ith transmit antenna to a jth receiveantenna, and S[n, k] denotes an nth symbol in a k-th sub-carrier. Asnoted from Equation (4), if H_(ij) is approximately equal to −H_(2j),the SNR of the received signal is very low.

Another conventional preamble transmitting/receiving scheme forMIMO-OFDM frame synchronization utilizes a direct modulated orthogonalpoly-phase sequence.

A direct modulated orthogonal poly-phase sequence is a chirp-likesequence used to form a preamble sequence. If P is a prime number, thedirect modulated orthogonal poly-phase sequence is comprised of (P−1)orthogonal sequences. Its excellent cross-correlation property is givenas

$\begin{matrix}{{{\Phi(k)} = {{\sum\limits_{n = 0}^{p^{2} - 1}{s_{q,n}^{*} \cdot s_{q^{\prime},{({n + k})}_{p^{2}}}}} \leq {\frac{1}{p^{2}}\mspace{14mu}{for}\mspace{14mu}{all}\mspace{14mu} k}}},{q \neq q^{\prime}}} & (5)\end{matrix}$

According to the second preamble transmitting/receiving scheme, thetransmit antennas transmit the same preamble sequence having (P−1)orthogonal sequences. This scheme faces the following problems:

(1) Although the length of the direct modulated orthogonal poly-phasesequence is the square of a prime number, the length of an OFDM framemust generally be a power of 2, for example, 64, 128, 256, . . . ; and

(2) While an ideal frame must be acquired at each point, it isimpossible to reduce the complex multiplications required and thusconsiderably greater computation is required.

Now, known preamble transmitting/receiving schemes for MIMO-OFDM channelestimation will be described below.

A first preamble transmitting/receiving scheme for MIMO-OFDM channelestimation is Geoffrey Li's single-symbol optimal training technique.FIG. 2 illustrates a preamble structure according to the first preambletransmitting/receiving scheme for MIMO-OFDM channel estimation.

Referring to FIG. 2, given Q transmit antennas, a first antennatransmits a preamble sequence S(t), and each of the other antennatransmits a preamble sequence S(t−T/Q), . . . , or S{t−(Q−1)T/Q}produced by rotating a preamble sequence for the previous antenna apredetermined number of symbols, that is, T/Q symbols. Q=Floor(N/L₀) inwhich N is the number of sub-carriers and L₀ is the maximum time delayspread of a sub-channel. Floor( ) is a function of obtaining an integerand T is the period of the preamble sequence. T is the product of thenumber of symbols included in the preamble sequence, N, and a symbolperiod T_(s).

A received signal at the jth receive antenna is determined by

$\begin{matrix}{{r_{j}\lbrack {n,k} \rbrack} = {{\sum\limits_{i}{{H_{ij}\lbrack {n,k} \rbrack}{S\lbrack {n,k} \rbrack}W_{N}^{k \cdot L_{0}}}} + {n_{j}\lbrack {n,k} \rbrack}}} & (6)\end{matrix}$where W_(N) represents an N-point fast Fourier transform (FFT). If p[n,k]=r[n, k]*S*[n, k], Equation (6) is expressed as

$\begin{matrix}{{P_{j}\lbrack {n,k} \rbrack} = {{\sum\limits_{i}{{H_{ij}\lbrack {n,k} \rbrack}W_{N}^{{- k} \cdot L_{0}}}} + {{n_{j}\lbrack {n,k} \rbrack} \cdot {S^{*}\lbrack {n,k} \rbrack}}}} & (7)\end{matrix}$

FIG. 3 illustrates an example of the time-domain channel responsecharacteristics of P_(j)[n, k]. Referring to FIG. 3, h_(0j) is a channelresponse characteristic from a first transmit antenna to a receiver,h_(1j) is a channel response characteristic from a second transmitantenna to the receiver, h_(2j) is a channel response characteristicfrom a third transmit antenna to the receiver, and h_(3j) is a channelresponse characteristic from a fourth transmit antenna to the receiver.Preamble sequences transmitted from the transmit antennas experiencechannels having different characteristics. The time-domain size T/Q ofthe channels varies with the number of the transmit antennas Q.

A mean square error (MSE) in the single-symbol optimal trainingtechnique is calculated by

$\begin{matrix}{{M\; S\; E} = {\frac{L_{0}}{N} \cdot \sigma_{n}^{2}}} & (8)\end{matrix}$wherein, σ_(n•)−σ_(n) indicates a noise power.

In accordance with the first preamble transmitting/receiving scheme forMIMO-OFDM channel estimation, although a preamble sequence istransmitted on all sub-carriers, only one training sequence structuresuffices. However, due to the rotation of a training sequence by apredetermined number of symbols for each transmit antenna, the number oftransmit antennas is limited by the number of the rotated symbols andthe length of the training sequence.

A second preamble transmitting/receiving scheme for MIMO-OFDM channelestimation utilizes Cordon L. Stuber and Apurva N. Mody's space-timecoding. In this scheme, known symbols are orthogonally transmitted inthe space domain through inversion and conjugation according to time andspace, namely according to transmit antennas. A preamble sequence for a2×2 system using two transmit antennas and two receive antennas isformed by

$\begin{matrix}\begin{bmatrix}S_{1} & S_{2} \\{- S_{2}^{*}} & S_{1}^{*}\end{bmatrix} & (9)\end{matrix}$

The above matrix means that symbols S₁ and S₂ are sequentiallytransmitted from a first transmit antenna and symbols S₂* and S₁* aresequentially transmitted from a second transmit antenna.

For a 4×4 system, a preamble sequence can be formed by

$\begin{matrix}\begin{bmatrix}S_{1} & S_{1} & S_{1} & S_{1} \\{- S_{2}} & S_{1} & {- S_{4}} & S_{3} \\{- S_{3}} & S_{4} & S_{1} & {- S_{2}} \\{- S_{4}} & {- S_{2}} & S_{2} & S_{1}\end{bmatrix} & (10)\end{matrix}$

FIG. 4 illustrates transmission/reception of a preamble sequenceaccording to the second preamble transmitting/receiving scheme forMIMO-OFDM channel estimation.

Referring to FIG. 4, Q preamble sequences, each having Q symbols areprovided to Q transmit antennas from time t to time t+(Q−1)T_(s) throughQ OFDM modulators. T_(s) is a symbol duration. The preamble sequencesarrive at L receive antennas on Q×L sub-channels having channel responsecharacteristics h₁₁ to h_(QL). L OFDM demodulators collect signals R₁ toR_(QL) received at the L receive antennas from time t to timet+(T−1)T_(s) and form a Q×L received signal matrix.

In the second preamble transmitting/receiving scheme, the minimum numberof training symbols needed for each transmit antenna is equal to thenumber of transmit antennas. As more training symbols are used, thepreamble sequences are longer. This is not feasible for burst orhigh-mobility communications.

SUMMARY OF THE INVENTION

An object of the present invention is to substantially solve at leastthe above problems and/or disadvantages and to provide at least theadvantages below. Accordingly, an object of the present invention is toprovide an effective preamble sequence structure and an effectivepreamble sequence transmitting method in a MIMO-OFDM system.

Another object of the present invention is to provide a method andapparatus for generating a preamble of a multi-symbol space-timestructure in a MIMO-OFDM system.

The above objects are achieved by a method and apparatus fortransmitting a preamble for frame synchronization and channel estimationin a MIMO-OFDM communication system. An OFDM communication system usingQ transmit antennas generates a base preamble sequence including acyclic prefix (CP) and an orthogonal sequence, generates a preamblesequence for each of the Q transmit antennas by rotating the orthogonalsequence by a predetermined number of symbols, and at least twicetransmits the generated preamble sequences from the Q transmit antennas.

If Q≦a predetermined number M, a preamble sequence for a kth antenna isS(t−(k−1)T/M). If Q>M and k≦M, the preamble sequence transmitted for thekth antenna is S(t−(k−1)T/M). If Q>M and k>M, the preamble sequence forthe kth antenna is (−1)^((PS-1))S(t−(k−M−1)T/M). Here, S(t) is theorthogonal sequence, T is the period of the orthogonal sequence, and PSis an index indicating a transmission period of the preamble sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates the structure of an OFDM frame including a preamblesequence in a typical OFDM communication system;

FIG. 2 illustrates the structure of a preamble according to aconventional preamble transmitting/receiving scheme for MIMO-OFDMchannel estimation;

FIG. 3 illustrates an example of time-domain channel responsecharacteristics of P_(j)[n, k];

FIG. 4 illustrates transmission/reception of a preamble sequenceaccording to another conventional preamble transmitting/receiving schemefor MIMO-OFDM channel estimation;

FIG. 5 is a simplified block diagram of a typical MIMO system;

FIG. 6 is a block diagram of a transmitter in a MIMO-OFDM system towhich the present invention is applied;

FIG. 7 is a block diagram of a receiver in the MIMO-OFDM system to whichthe present invention is applied;

FIG. 8 illustrates an embodiment of a preamble structure according tothe present invention;

FIG. 9 illustrates the transmission of preambles illustrated in FIG. 8;

FIG. 10 illustrates the results of frame synchronization according tothe present invention;

FIG. 11 illustrates an embodiment of a preamble structure for a 4×4 MIMOsystem according to the present invention;

FIG. 12 illustrates an embodiment of a preamble structure for a 6×6 MIMOsystem according to the present invention;

FIG. 13 illustrates preambles illustrated in FIG. 12 in matrix blocks;and

FIG. 14 is a graph illustrating channel estimation gain with respect toMSE in a multi-channel WLAN (Wireless Local Access Network) system usingthe preamble structure of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

A MIMO-OFDM system to which the present invention is applied will firstbe described below.

FIG. 5 is a simplified block diagram of a typical MIMO system. Referringto FIG. 5, Q×L sub-channels 30 are defined between a transmitter 10having Q transmit antennas and a receiver 20 having L receive antennas.The sub-channels 30 each have a unique channel response characteristich_(ql) and these characteristics are expressed as a Q×L channel matrixH_(QL).

FIG. 6 is a block diagram of a transmitter in a MIMO-OFDM system towhich the present invention is applied. The transmitter transmits thesame user information through a plurality of transmit antennas toachieve an antenna diversity gain.

Referring to FIG. 6, an encoder (ENC) 102 generates a coded sequence byencoding an information sequence S(t) at a predetermined code rate. Ademultiplexer (DEMUX) 104 distributes the coded sequence to a pluralityof interleavers (INTs) 106 to 114 corresponding to transmit antennas 112to 120. The interleavers 106 to 114 each interleave the input bits.Mappers (MAPs) 108 to 116 each map the interleaved bits to modulationsymbols according to a mapping rule, for example, PSK (Phase ShiftKeying) or QAM (Quadrature Amplitude Modulation).

OFDM modulators (MODs) 110 to 118 each generate an OFDM symbol byinserting a pilot symbol for every predetermined number of modulationsymbols, generate an OFDM frame by adding a preamble sequence havingknown symbols at the start of a predetermined number of OFDM symbols,and inverse-fast-Fourier-transform (IFFT) the OFDM frame. The IFFT OFDMframes are transmitted through their corresponding transmit antennas 112to 120 through an RF (Radio Frequency) module (not shown).

FIG. 7 is a block diagram of a receiver in the MIMO-OFDM system to whichthe present invention is applied. The receiver is a counterpart to thetransmitter illustrated in FIG. 6.

Referring to FIG. 7, signals received at receive antennas 202 to 216 areapplied to the inputs of OFDM demodulators (DEMODs) 204 to 218 throughan RF module (not shown). The OFDM demodulators 204 to 218 eachdistinguish a preamble from OFDM symbols on a frame-by-frame basis,accurately acquire frame synchronization by detecting the preamble, andgenerate a plurality of modulation symbols by fast-Fourier-transformingthe signal. While not shown, the detected preamble is used in a channelestimator that estimates channel response characteristics from thetransmitter to the receiver.

Demappers (DEMAPs) 206 to 216 each demap received modulation symbolsaccording to a demapping rule corresponding to the mapping rule used inthe transmitter. Deinterleavers (DEINTs) 208 to 216 each deinterleavedemapped bits according to a deinterleaving rule corresponding to theinterleaving rule used in the transmitter. A multiplexer (MUX) 212multiplexes the deinterleaved bits and a decoder 210 recovers theinformation sequence S(t) by decoding the multiplexed bits at the coderate used in the transmitter.

In the MIMO-OFDM system having the above configuration, a preamblesequence consists of special symbols generated by an OFDM modulator andattached to an OFDM frame to indicate the start of the OFDM frame. Amobile station must synchronize to the start point of the data toreceive the data. For this purpose, the mobile station acquires apreamble sequence commonly used in the entire system before receivingthe data.

The preamble sequence is used for frame synchronization, frequencysynchronization (i.e. frequency offset estimation), and channelestimation. The OFDM communication system estimatestime/frequency/channel information using the preamble sequence at thestart of each frame or data burst, and updates thetime/frequency/channel information using a cyclic prefix (CP), insertedto avoid inter-symbol interference, and pilot symbols inserted betweenmodulation symbols.

As known, frame synchronization is performed in two stages: coarse framesynchronization and fine frame synchronization.

The coarse frame synchronization is the process of detecting the startpoint of an OFDM frame by sampling in an approximate range. Thecorrelation peak of a CP is used for the coarse frame synchronization.The following equation represents a metric for the coarse framesynchronization

$\begin{matrix}{\phi_{n} = {{\sum\limits_{k = 0}^{G - 1}( {r_{j,{n + k}}^{*} \cdot r_{j,{n + k + N}}} )}}^{2}} & (11)\end{matrix}$where G denotes the window size of the frame synchronization, r_(j, x)denotes an xth signal in a sequence received at a jth receive antenna,and N denotes the length of the sequence. Thus, a coarse frame startpoint is a time index n that maximizes φ_(n).

The coarse frame synchronization reduces the range of fine framesynchronization. The computation range of Equation (12) is narrowcompared to that of Equation (2), in calculating the cross-correlationproperty for the fine frame synchronization

$\begin{matrix}{{{\phi(k)} = {{\sum\limits_{n = 0}^{N - 1}{s_{q,n}^{*} \cdot s_{q^{\prime},{({n + k})}_{N}}}} = {{0\mspace{14mu}{for}\mspace{14mu}{all}\mspace{14mu} k} \in K_{catch}}}},{q \neq q^{\prime}}} & (12)\end{matrix}$where s_(q, n) denotes an nth data symbol in a sequence transmitted froma qth transmit antenna and K_(catch) denotes the range of the fine framesynchronization. Thus, the frame start point is a time index k thatmakes the fine frame synchronization metric φ(k) zero.

An embodiment of a preamble sequence structure design in a multi-channelWLAN system according to the present invention will now be described.

Let a root mean square (RMS) delay be equal to 50 ns, a sampling time beequal to 25 ns, a CP length be equal to 32 points, and the total lengthof data be equal to 128 points. The length of valid data in the data is112 points, and the DC (Direct Current) and edge components in a signalfrequency band are nulls. Here, a 2×2 MIMO system using two transmitantennas and two receive antennas is used as an example. A point refersto the position of a sub-carrier subject to N-point FFT. For example, ifa CP is 32 points long, this implies that the CP is transmitted on 32sub-carriers.

First of all, orthogonal sequences are generated using an extended CAZAC(Constant Amplitude Zero Auto-Correlation) sequence.

For example, a base CAZAC sequence is1, 1, 1, 1 , 1, j, −1, −j, 1, −1, 1, −1, 1, −j, −1, j  (13)

By inserting three zeroes between every adjacent pair of elements in thebase CAZAC sequence, the following sequence is generated1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, j, 0, 0, 0,−1, 0, 0, 0,−j, 0, 0, 0, 1, 0, 0, 0, −1, 0, 0, 0, 1, 0, 0, 0, −1, 0, 0,0, 1, 0, 0, 0, −j, 0, 0, 0, −1, 0, 0, 0, j  (14)The peak-to-average power ratio of the above extended CAZAC sequence is6 dB.

The above orthogonal sequence is converted to the frequency domain, forspectrum shaping. The resulting new sequence is again converted to thetime domain, to thereby create a preamble sequence.

Thus, the preamble structure according to the present invention is givenas illustrated in Table 1 below.

TABLE 1 CP0 S₆₄[1:64] S₆₄[1:64] Antenna 0 CP1 S₆₄[33:64] S₆₄[33:64]Antenna 1 S₆₄[1:32] S₆₄[1:32]

FIG. 8 illustrates the preamble structure according to an embodiment ofthe present invention, and FIG. 9 illustrates transmission of preamblesillustrated in FIG. 8. As stated earlier, the illustrated preamblestructure is for the 2×2 MIMO system.

Referring to FIG. 8, a first antenna (antenna 1) transmits a sequence of64 bits, S[1:64] for a first transmission period and a second antenna(antenna 2) transmits a 32-bit rotated version of the sequence,S[33:64]S[1:32]. 32 bits is the quotient of dividing the sequencelength, 64, by the number of the transmit antennas, 2. These sequencesare repeatedly transmitted for a second transmission period.Transmission of a 64-bit sequence is equivalent to the use of 64sub-carriers. Therefore, as illustrated in FIG. 9, the first antennatransmits the input sequence on sub-carriers #0 to #63, while the secondantenna transmits the input sequence on sub-carriers #32 to #31.

Then, the receiver cross-correlates the extended CAZAC sequence withreceived complex symbols, thereby performing the fine framesynchronization by

$\begin{matrix}{{\Phi_{n} = {\sum\limits_{q = 1}^{Q}\frac{{\phi_{q,n}}^{2}}{( P_{n}^{\prime} )^{2}}}}{where}{\phi_{q,n} = {\sum\limits_{k = 0}^{N - 1}( {s_{q,k}^{*} \cdot r_{j,{n + k}}} )}}{P_{n}^{\prime} = {{\sum\limits_{k = 0}^{N - 1}{r_{j,{n + k}}}^{2}} = {constant}}}} & (15)\end{matrix}$where N is the length of the preamble sequence according to the presentinvention, Q is the number of the transmit antennas, s_(q, k) is a kthsymbol in a preamble sequence transmitted from a qth transmit antenna,and r_(j, n+k) is an (n+k)th signal in a preamble sequence received at ajth receive antenna.

Similarly, the start point of the frame is determined as a time point nwhere Φ_(n)=0.

Since time index n in the fine frame synchronization indicates an FFTpoint, full complex multiplications will increase complexityconsiderably. However, with the use of the CAZAC sequence of a simplestructure according to the present invention, only addition andswitching will suffice.

With the sequence rotation of the present invention, a received signalis determined byr _(j)(k)=H _(0j)(k)S(k)+H _(1j)(k)·(−1)^(k) S(k)+n _(j)(k)  (16)

Even if channels are correlated, it is impossible to reduce SNR in thesystem. Yet, simulation results reveal that the present invention isrobust compared to the conventional technology in which the samesequence is applied to all antennas.

FIG. 10 illustrates the results of frame synchronization according tothe present invention. Changes over time in coarse and fine framesynchronization metrics are illustrated. In FIG. 10, time points havingthe highest metric values are conspicuous in the fine framesynchronization.

While each transmit antenna transmits the same preamble sequence for twotransmission periods as illustrated in FIG. 8 according to theembodiment of the present invention, it can be further contemplated asanother embodiment that each transmit antenna transmits the samesequence for more than two transmission periods to allow more stableframe synchronization and more accurate channel estimation.

FIG. 11 illustrates an embodiment of a preamble structure for a 4×4 MIMOsystem according to the present invention.

Referring to FIG. 11, for a first transmission period, a first antenna(antenna 0) transmits an extended CAZAC sequence S(t) and a secondantenna (antenna 1) transmits a T/4-symbol rotated version of S(t),S(t−T/4). T denotes the period of the sequence. In the same manner,third and fourth antennas (antenna 2 and antenna 3) transmit T/2-symboland 3T/4-symbol rotated versions of S(t), S(t−T/2) and S(t−3T/4),respectively. Each transmit antenna repeatedly transmits the samesequence for two or more transmission periods.

In general, a preamble structure for Q transmit antennas is given asillustrated in Table 2. In Table 2, PS denotes the index of atransmission period for the preamble sequence.

TABLE 2 PS 1 2 . . . Antenna 1 S(t) S(t) . . . Antenna 2 S(t − T/Q) S(t− T/Q) . . . . . . . . . . . . . . . Antenna k S(t − (k − 1)T/Q) S(t −(k − 1)T/Q) . . . . . . . . . . . . . . . Antenna Q S(t − (Q − 1)T/Q)S(t − (Q − 1)T/Q) . . .

Meanwhile, if Q is greater than a predetermined number M, an (M+1)th tothe last antenna cyclically transmits the sequences set for the first toMth antennas. The preamble structure is created by repeating thosesequences set for the first to Mth antennas in the space domain.

The CP length is determined by the range of frame synchronization. Thus,the maximum available number of transmit antennas, M is floor (N/L₀). L₀is the maximum time delay spread of a sub-channel. If Q is greater thanM, a preamble structure is created by repeating those sequences set forthe first to Mth antennas in the space domain and an (M+1)th to the lastantenna cyclically transmits the sequences set for first to Mthantennas. Also, to ensure robust channel estimation, the preamblestructure is orthogonally designed in the time domain.

For example, if M=4 and Q=6, a preamble structure is given asillustrated in Table 3.

TABLE 3 PS 1 2 3 . . . Antenna 1 S(t) S(t) S(t) . . . Antenna 2 S(t −T/4) S(t − T/4) S(t − T/4) . . . Antenna 3 S(t − T/2) S(t − T/2) S(t −T/2) . . . Antenna 4 S(t − 3T/4) S(t − 3T/4) S(t − 3T/4) . . . Antenna 5S(t) −S(t) S(t) . . . Antenna 6 S(t − T/4) −S(t − T/4) S(t − T/4) . . .

FIG. 12 illustrates a preamble structure for a 6×6 MIMO system accordingto an embodiment of the present invention.

Referring to FIG. 12, for a first transmission period, a first antenna(antenna 0) transmits the extended CAZAC sequence S(t). Second, thirdand fourth antennas (antenna 1, antenna 2 and antenna 3) respectivelytransmit S(t−T/4), S(t−T/2) and S(t−3T/4) produced by rotating S(t) byT/4, T/2 and 3T/4, respectively. Fifth and sixth antennas (antenna 4 andantenna 5) transmit the sequences S(t) and S(t−T/4), respectively,starting from S(t) again. For second, third and fourth transmissionperiods, each antenna repeatedly transmits the same sequence except thatthe fifth and sixth antennas transmit inverse sequences −S(t) and−S(t−T/4), respectively, for the second and fourth transmission periodsin order to ensure orthogonality in the time domain.

In the above case, a matrix block

$\quad\begin{bmatrix}A & A \\A & {- A}\end{bmatrix}$ensures the time-domain orthogonality. The rows of the matrix blockrepresent antenna groups each having M transmit antennas and the columnsrepresent sequence periods. Therefore, for M=4, one element. A canaccommodate up to four antennas. This preamble structure can support upto eight transmit antennas when M=4.

FIG. 13 illustrates preambles illustrated in FIG. 12 in matrix blocks.Referring to FIG. 13, an element A in a matrix block for the 6×6 MIMOsystem is

$\begin{bmatrix}{S(t)} \\{S( {t - {T/M}} )} \\\vdots \\{S( {t - {( {M - 1} ){T/M}}} )}\end{bmatrix}.$

Each antenna repeats a corresponding row of the element A or −A in thetime domain. In this case, despite increased complexity, the accuracy offrame synchronization and channel estimation is improved.

To generalize, given Q transmit antennas (Q>M), antennas in differentantenna groups transmit different preamble sequences. A kth antenna(k≦M) in a first antenna group having first to Mth antennas transmits apreamble sequence S(t−(k−1)T/M), whereas a k′th antenna (k′>M) in asecond antenna group having (M+1)th to the last antennas transmits apreamble sequence (−1)^((PS-1))S(t−(k−M−1)T/M). Each antenna transmitsthe same preamble sequence repeatedly for two or more transmissionperiods.

FIG. 14 is a graph illustrating channel estimation gain with respect toMSE in a multi-channel WLAN system using the preamble structure of thepresent invention. * denotes MSE versus SNR in a conventional preamblestructure and + denotes MSE versus SNR in the inventive preamblestructure. As noted from FIG. 14, the inventive preamble structureoffers less MSEs over all SNRs.

As described above, the preamble structure of the present inventionflexibly controls the length of a preamble. Therefore, it is feasiblefor burst and high-mobility communications. Also, repetition ofsequences in the time domain leads to a very excellent performance forframe synchronization and clock offset synchronization.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A method of receiving a preamble in an orthogonal frequency divisionmultiplexing (OFDM) communication system using Q receiving antennas,comprising the steps of: receiving a preamble, the preamble beinggenerated by: generating a base preamble sequence including a cyclicprefix (CP) and an orthogonal sequence; and generating a preamblesequence for each of the Q receiving antennas from the base preamblesequence by rotating the orthogonal sequence of the base preamblesequence by a different predetermined number of symbols, and at leasttwice receiving the generated preamble sequences, such that the preamblesequence received at a kth antenna is (−1)^((PS-1))S(t−(k−M−1)T/M),wherein M is a predetermined number, k is greater than M, S(t) is theorthogonal sequence, T is the period of the orthogonal sequence, and PSis an index indicating a transmission period of the preamble sequence.2. The method of claim 1, wherein generating a preamble sequencecomprises, if Q is less than or equal to the predetermined number M,generating a preamble sequence for each of the receiving antennas suchthat a preamble sequence received from the kth antenna is S(t−(k−1)T/M),where S(t) is the orthogonal sequence and T is the period of theorthogonal sequence.
 3. A method of receiving a preamble in anorthogonal frequency division multiplexing (OFDM) communication systemusing Q receiving antennas, the method comprising: receiving a preamble,the preamble being generated by: generating a base preamble sequenceincluding a cyclic prefix (CP) and an orthogonal sequence; andgenerating a preamble sequence for each of the Q receiving antennas fromthe base preamble sequence by rotating the orthogonal sequence of thebase preamble sequence by a different predetermined number of symbols,and at least twice receiving the generated preamble sequences, whereingenerating a preamble sequence comprises: if Q is greater than M,generating a preamble sequence for each of the receiving antennas suchthat if k is less than or equal to M, the preamble sequence received atthe kth antenna is S(t−(k−1)T/M), and if k is greater than M, thepreamble sequence received from the kth antenna is(−1)^((PS-1))S(t−(k−M−1)T/M), and wherein S(t) is the orthogonalsequence, T is the period of the orthogonal sequence, and PS is an indexindicating a transmission period of the preamble sequence.
 4. The methodof claim 2, wherein M is an integer part of the quotient calculated bydividing the length N of the preamble sequence by a maximum delay spreadL₀ of a sub-channel.
 5. The method of claim 3, wherein M is an integerpart of the quotient calculated by dividing the length N of the preamblesequence by a maximum delay spread L₀ of a sub-channel.
 6. The method ofclaim 2, wherein M is
 4. 7. The method of claim 3, wherein M is
 4. 8.The method of claim 1, wherein the orthogonal sequence is an extendedCAZAC (Constant Amplitude Zero Auto-Correlation) sequence.
 9. A methodof receiving a preamble in an orthogonal frequency division multiplexing(OFDM) communication system using Q receiving antennas, the methodcomprising: receiving a preamble, the preamble being generated by:generating a base preamble sequence including a cyclic prefix (CP) andan orthogonal sequence; and generating a preamble sequence for each ofthe Q receiving antennas from the base preamble sequence by rotating theorthogonal sequence of the base preamble sequence by a differentpredetermined number of symbols, and at least twice receiving thegenerated preamble sequences, wherein the orthogonal sequence is anextended CAZAC (Constant Amplitude Zero Auto-Correlation) sequence andthe extended CAZAC sequence is 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0,0, 0, 1, 0, 0, 0, j, 0, 0, 0, −1, 0, 0, 0, −j, 0, 0, 0, 1, 0, 0, 0, −1,0, 0, 0, 1, 0, 0, 0, −1, 0, 0, 0, 1, 0, 0, 0,−j, 0, 0, 0, −1, 0, 0, 0,j.
 10. A method of receiving a preamble in an orthogonal frequencydivision multiplexing (OFDM) communication system using a plurality ofreceiving antennas, the method comprising: receiving a preamble sequencehaving a cyclic prefix (CP) and an orthogonal sequence S(t) of period T;for a first sequence transmission period: receiving, through a firstreceiving antenna, a first preamble sequence having the CP and the S(t);and receiving, through a second receiving antenna, a second preamblesequence having the CP and a S(t−T/2); and for a second sequencetransmission period: receiving, through the first receiving antenna, thefirst preamble sequence; and receiving, through the second receivingantenna, the second preamble sequence, such that the preamble sequencereceived at a kth antenna is (−1)^((PS-1))S(t−(k−M−1)T/M), wherein M isa predetermined number, k is greater than M, S(t) is the orthogonalsequence, T is the period of the orthogonal sequence, and PS is an indexindicating a transmission period of the preamble sequence.
 11. A methodof receiving a preamble in an orthogonal frequency division multiplexing(OFDM) communication system using a plurality of receiving antennas, themethod comprising: receiving a preamble sequence having a cyclic prefix(CP) and an orthogonal sequence S(t) of period T; for a first sequencetransmission period: receiving, through a first receiving antenna, afirst preamble sequence having the CP and the S(t); receiving, through asecond receiving antenna a second preamble sequence having the CP and aS(t−T/4); receiving, through a third receiving antenna, a third preamblesequence having the CP and a S(t−T/2); and receiving, through a fourthreceiving antenna, a fourth preamble sequence having the CP and aS(t−3T/4); and for a second sequence transmission period, receiving, thefirst to fourth preamble sequences through the first to fourth receivingantennas, respectively, such that the preamble sequence received at akth antenna is (−1)^((PS-1))S(t−(k−M−1)T/M), wherein M is apredetermined number, k is greater than M, S(t) is the orthogonalsequence, T is the period of the orthogonal sequence, and PS is an indexindicating a transmission period of the preamble sequence.
 12. A methodof receiving a preamble in an orthogonal frequency division multiplexing(OFDM) communication system using six receicing antennas, the methodcomprising: generating a preamble sequence having a cyclic prefix (CP)and an orthogonal sequence S(t) of period T; for a first sequencetransmission period: receiving, through a first receiving antenna, afirst preamble sequence having the CP and the S(t); receiving through asecond receiving antenna, a second preamble sequence having the CP and aS(t−T/4); receiving through a third receiving antenna, a third preamblesequence having the CP and a S(t−T/2); receiving through a fourthreceiving antenna, a fourth preamble sequence having the CP and aS(t−3T/4); receiving through a fifth receiving antenna, the firstpreamble sequence; and receiving through a sixth receiving antenna, thesecond preamble sequence; and for a second sequence transmission period:receiving, through the first receiving antenna, the first preamblesequence; receiving, through the second receiving antenna, the secondpreamble sequence; receiving, through the third receiving antenna, thethird preamble sequence; receiving through the fourth receiving antennathe fourth preamble sequence; receiving through the fifth receivingantenna a fifth preamble sequence having the CP and a −S(t); andreceiving through the sixth receiving antenna a sixth preamble sequencehaving the CP and a −S(t−T/4).
 13. The method of claim 10, wherein thesecond sequence transmission period is a next sequence transmissionperiod following the first sequence transmission period.
 14. The methodof claim 11, wherein the second sequence transmission period is a nextsequence transmission period following the first sequence transmissionperiod.
 15. An apparatus for receiving a preamble in an orthogonalfrequency division multiplexing (OFDM) communication system, theapparatus comprising: Q receiving antennas, each configured to receive apreamble sequence generated by: generating a base preamble sequenceincluding a cyclic prefix (CP) and an orthogonal sequence; andgenerating a preamble sequence for each of the Q receiving antennas fromthe base preamble sequence by rotating the orthogonal sequence of thebase preamble sequence by a different predetermined number of symbols,and at least twice receiving the generated preamble sequences, such thatthe preamble sequence received from a kth antenna is(−1)^((PS-1))S(t−(k−M−1)T/M), wherein M is a predetermined number, k isgreater than M, S(t) is the orthogonal sequence, T is the period of theorthogonal sequence, and PS is an index indicating a transmission periodof the preamble sequence.
 16. The method of claim 15, wherein generatinga preamble sequence comprises, if Q is less than or equal to thepredetermined number M, generating a preamble sequence for each of thereceiving antennas such that a preamble sequence received from the kthantenna is S(t−(k−1)T/M), where S(t) is the orthogonal sequence and T isthe period of the orthogonal sequence.
 17. An apparatus for receiving apreamble in an orthogonal frequency division multiplexing (OFDM)communication system, the apparatus comprising: Q receiving antennas,each configured to receive a preamble sequence generated by: generatinga base preamble sequence including a cyclic prefix (CP) and anorthogonal sequence; and generating a preamble sequence for each of theQ receiving antennas from the base preamble sequence by rotating theorthogonal sequence of the base preamble sequence by a differentpredetermined number of symbols, and at least twice receiving thegenerated preamble sequences, wherein generating a preamble sequencecomprises: if Q is greater than M, generating a preamble sequence foreach of the receiving antennas such that if k is less than or equal toM, the preamble sequence received from the kth antenna is S(t−(k−1)T/M),and if k is greater than M, the preamble sequence received from the kthantenna is (−1)^((PS-1))S(t−(k−M−1)T/M), and wherein S(t) is theorthogonal sequence, T is the period of the orthogonal sequence, and PSis an index indicating a transmission period of the preamble sequence.18. The apparatus of claim 16, wherein M is an integer part of thequotient calculated by dividing the length N of the preamble sequence bya maximum delay spread L₀ of a sub-channel.
 19. The apparatus of claim17, wherein M is an integer part of the quotient calculated by dividingthe length N of the preamble sequence by a maximum delay spread L₀ of asub-channel.
 20. The apparatus of claim 16, wherein M is
 4. 21. Theapparatus of claim 17, wherein M is
 4. 22. The apparatus of claim 15,wherein the orthogonal sequence is an extended CAZAC (Constant AmplitudeZero Auto-Correlation) sequence.
 23. An apparatus for receiving apreamble in an orthogonal frequency division multiplexing (OFDM)communication system, the apparatus comprising: Q receiving antennas,each configured to receive a preamble sequence generated by: generatinga base preamble sequence including a cyclic prefix (CP) and anorthogonal sequence; and generating a preamble sequence for each of theQ receiving antennas from the base preamble sequence by rotating theorthogonal sequence of the base preamble sequence by a differentpredetermined number of symbols, and at least twice receiving thegenerated preamble sequences, wherein the orthogonal sequence is anextended CAZAC (Constant Amplitude Zero Auto-Correlation) sequence andthe extended CAZAC sequence is 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0,0, 0, 1, 0, 0, 0, j, 0, 0, 0, −1, 0, 0, 0,−j, 0, 0, 0, 1, 0, 0, 0, −1,0, 0, 0, 1, 0, 0, 0, −1, 0, 0, 0, 1, 0, 0, 0,−j, 0, 0, 0, −1, 0, 0, 0,j.
 24. An apparatus for receiving a preamble in an orthogonal frequencydivision multiplexing (OFDM) communication system, the apparatuscomprising: a plurality of receiving antennas, each configured toreceive a preamble sequence generated by: generating a preamble sequencehaving a cyclic prefix (CP) and an orthogonal sequence S(t) of period T;for a first sequence transmission period: receiving, through a firstreceiving antenna, a first preamble sequence having the CP and the S(t);and receiving, through a second receiving antenna, a second preamblesequence having the CP and a S(t−T/2); and for a second sequencetransmission period: receiving, through the first receiving antenna, thefirst preamble sequence; and receiving, through the second receivingantenna, the second preamble sequence, such that the preamble sequencereceived from a kth antenna is (−1)^((PS-1))S(t−(k−M−1)T/M), wherein Mis a predetermined number, k is greater than M, S(t) is the orthogonalsequence, T is the period of the orthogonal sequence, and PS is an indexindicating a transmission period of the preamble sequence.
 25. Anapparatus for receiving a preamble in an orthogonal frequency divisionmultiplexing (OFDM) communication system, the apparatus comprising: aplurality of receiving antennas, each configured to receive a preamblesequence generated by: generating a preamble sequence having a cyclicprefix (CP) and an orthogonal sequence S(t) of period T; for a firstsequence transmission period: receiving, through a first receivingantenna, a first preamble sequence having the CP and the S(t);receiving, through a second receiving antenna, a second preamblesequence having the CP and a S(t−T/4); receiving, through a thirdreceiving antenna, a third preamble sequence having the CP and aS(t−T/2); and receiving, through a fourth receiving antenna, a fourthpreamble sequence having the CP and a S(t−3T/4); and for a secondsequence transmission period, receiving the first to fourth preamblesequences through the first to fourth receiving antennas, respectively,such that the preamble sequence received from a kth antenna is(−1)^((PS-1))S(t−(k−M−1)T/M), wherein M is a predetermined number, k isgreater than M, S(t) is the orthogonal sequence, T is the period of theorthogonal sequence, and PS is an index indicating a transmission periodof the preamble sequence.
 26. An apparatus for receiving a preamble inan orthogonal frequency division multiplexing (OFDM) communicationsystem, the apparatus comprising: six receiving antennas, eachconfigured to receive a preamble sequence generated by: generating apreamble sequence having a cyclic prefix (CP) and an orthogonal sequenceS(t) of period T; for a first sequence transmission period: receiving,through a first receiving antenna, a first preamble sequence having theCP and the S(t); receiving, through a second receiving antenna, a secondpreamble sequence having the CP and a S(t−T/4); receiving, through athird receiving antenna, a third preamble sequence having the CP and aS(t−T/2); receiving, through a fourth receiving antenna, a fourthpreamble sequence having the CP and a S(t−3T/4); receiving, through afifth receiving antenna, the first preamble sequence; and receiving,through a sixth receiving antenna, the second preamble sequence; and fora second sequence transmission period: receiving, through the firstreceiving antenna, the first preamble sequence; receiving, through thesecond receiving antenna, the second preamble sequence; receiving,through the third receiving antenna, the third preamble sequence;receiving, through the fourth receiving antenna, the fourth preamblesequence; receiving, through the fifth receiving antenna, a fifthpreamble sequence having the CP and a −S(t); and receiving, through thesixth receiving antenna, a sixth preamble sequence having the CP and a−S(t−T/4).
 27. The apparatus of claim 24, wherein the second sequencetransmission period is a next sequence transmission period following thefirst sequence transmission period.
 28. The apparatus of claim 25,wherein the second sequence transmission period is a next sequencetransmission period following the first sequence transmission period.29. An apparatus for transmitting a preamble in an orthogonal frequencydivision multiplexing (OFDM) communication system, the apparatuscomprising: Q transmit antennas; and a preamble generating deviceconfigured to: generate a base preamble sequence including a cyclicprefix (CP) and an orthogonal sequence; and generate a preamble sequencefor each of the Q transmit antennas from the base preamble sequence byrotating the orthogonal sequence of the base preamble sequence by adifferent predetermined number of symbols, and at least twicetransmitting the generated preamble sequences from the Q transmitantennas, such that the preamble sequence transmitted from a kth antennais (−1)^((PS-1))S(t−(k−M−1)T/M), wherein M is a predetermined number, kis greater than M, S(t) is the orthogonal sequence, T is the period ofthe orthogonal sequence, and PS is an index indicating a transmissionperiod of the preamble sequence.
 30. The apparatus of claim 29, whereinstep (2) comprises the step of, if Q is less than or equal to thepredetermined number M, generating a preamble sequence for each of thetransmit antennas such that a preamble sequence transmitted from the kthantenna is S(t−(k−1)T/M), where S(t) is the orthogonal sequence and T isthe period of the orthogonal sequence.
 31. An apparatus for transmittinga preamble in an orthogonal frequency division multiplexing (OFDM)communication system, the apparatus comprising: Q transmit antennas; anda preamble generating device configured to: generate a base preamblesequence including a cyclic prefix (CP) and an orthogonal sequence; andgenerate a preamble sequence for each of the Q transmit antennas fromthe base preamble sequence by rotating the orthogonal sequence of thebase preamble sequence by a different predetermined number of symbols,and at least twice transmitting the generated preamble sequences fromthe Q transmit antennas, wherein generating a preamble sequencecomprises: if Q is greater than M, generating a preamble sequence foreach of the transmit antennas such that if k is less than or equal to M,the preamble sequence transmitted from the kth antenna is S(t−(k−1)T/M),and if k is greater than M, the preamble sequence transmitted from thekth antenna is (−1)^((PS-1))S(t−(k−M−1)T/M), and wherein S(t) is theorthogonal sequence, T is the period of the orthogonal sequence, and PSis an index indicating a transmission period of the preamble sequence.32. The apparatus of claim 30, wherein M is an integer part of thequotient calculated by dividing the length N of the preamble sequence bya maximum delay spread L₀ of a sub-channel.
 33. The apparatus of claim31, wherein M is an integer part of the quotient calculated by dividingthe length N of the preamble sequence by a maximum delay spread L₀ of asub-channel.
 34. The apparatus of claim 30, wherein M is
 4. 35. Theapparatus of claim 31, wherein M is
 4. 36. The apparatus of claim 29,wherein the orthogonal sequence is an extended CAZAC (Constant AmplitudeZero Auto-Correlation) sequence.
 37. An apparatus for transmitting apreamble in an orthogonal frequency division multiplexing (OFDM)communication system using, the apparatus comprising: Q transmitantennas; and a preamble generating device configured to: generate abase preamble sequence including a cyclic prefix (CP) and an orthogonalsequence; and generate a preamble sequence for each of the Q transmitantennas from the base preamble sequence by rotating the orthogonalsequence of the base preamble sequence by a different predeterminednumber of symbols, and at least twice transmitting the generatedpreamble sequences from the Q transmit antennas, wherein the orthogonalsequence is an extended CAZAC (Constant Amplitude Zero Auto-Correlation)sequence and the extended CAZAC sequence is 1, 0, 0, 0, 1, 0, 0, 0, 1,0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, j, 0, 0, 0, −1, 0, 0, 0, −j, 0, 0, 0,1, 0, 0, 0, −1, 0, 0, 0, 1, 0, 0, 0, −1, 0, 0, 0, 1, 0, 0, 0, −j, 0, 0,0, −1, 0, 0, 0, j.
 38. An apparatus for transmitting a preamble in anorthogonal frequency division multiplexing (OFDM) communication system,the apparatus comprising: a plurality of transmit antennas; and apreamble generating device configured to: generate a preamble sequencehaving a cyclic prefix (CP) and an orthogonal sequence S(t) of period T;for a first sequence transmission period: transmit, through a first ofthe transmit antennas, a first preamble sequence having the CP and theS(t); and transmit, through a second of the transmit antennas, a secondpreamble sequence having the CP and a S(t−T/2); and for a secondsequence transmission period: transmit, through the first transmitantenna, the first preamble sequence; and transmit, through the secondtransmit antenna, the second preamble sequence, such that the preamblesequence transmitted from a kth antenna is (−1)^((PS-1))S(t−(k−M−1)T/M),wherein M is a predetermined number, k is greater than M, S(t) is theorthogonal sequence, T is the period of the orthogonal sequence, and PSis an index indicating a transmission period of the preamble sequence.39. An apparatus for transmitting a preamble in an orthogonal frequencydivision multiplexing (OFDM) communication system, the apparatuscomprising: a plurality of transmit antennas; and a preamble generatingdevice configured to: generate a preamble sequence having a cyclicprefix (CP) and an orthogonal sequence S(t) of period T; for a firstsequence transmission period: transmit, through a first of the transmitantennas, a first preamble sequence having the CP and the S(t);transmit, through a second of the transmit antennas, a second preamblesequence having the CP and a S(t−T/4); transmit, through a third of thetransmit antennas, a third preamble sequence having the CP and aS(t−T/2); and transmit, through a fourth of the transmit antennas, afourth preamble sequence having the CP and a S(t−3T/4); and for a secondsequence transmission period, transmit the first to fourth preamblesequences through the first to fourth transmit antennas, respectively,such that the preamble sequence transmitted from a kth antenna is(−1)^((PS-1))S(t−(k−M−1)T/M), wherein M is a predetermined number, k isgreater than M, S(t) is the orthogonal sequence, T is the period of theorthogonal sequence, and PS is an index indicating a transmission periodof the preamble sequence.
 40. An apparatus for transmitting a preamblein an orthogonal frequency division multiplexing (OFDM) communicationsystem using, the apparatus comprising: six transmit antennas; and apreamble generating device configured to: generating a preamble sequencehaving a cyclic prefix (CP) and an orthogonal sequence S(t) of period T;for a first sequence transmission period: transmit, through a first ofthe transmit antennas, a first preamble sequence having the CP and theS(t); transmit, through a second of the transmit antennas, a secondpreamble sequence having the CP and a S(t−T/4); transmit, through athird of the transmit antennas, a third preamble sequence having the CPand a S(t−T/2); transmit, through a fourth of the transmit antennas, afourth preamble sequence having the CP and a S(t−3T/4); transmit,through a fifth of the transmit antennas, the first preamble sequence;and transmit, through a sixth of the transmit antennas, the secondpreamble sequence; and for a second sequence transmission period:transmit, through the first transmit antenna, the first preamblesequence; transmit, through the second transmit antenna, the secondpreamble sequence; transmit, through the third transmit antenna, thethird preamble sequence; transmit, through the fourth transmit antenna,the fourth preamble sequence; transmit, through the fifth transmitantenna, a fifth preamble sequence having the CP and a −S(t); andtransmit, through the sixth transmit antenna, a sixth preamble sequencehaving the CP and a −S(t−T/4).
 41. The apparatus of claim 38, whereinthe second sequence transmission period is a next sequence transmissionperiod following the first sequence transmission period.
 42. Theapparatus of claim 39, wherein the second sequence transmission periodis a next sequence transmission period following the first sequencetransmission period.